1. Field of the Invention
This invention relates to zoom objectives, and, more particularly, to small-sized, wide angle zoom objectives having three lens units with the first two lens units counting from front made movable to effect zooming.
2. Description of the Prior Art
In the past, in order to achieve a reduction of the bulk and size of a zoom objective lens, such lens has been formed by a diverging 1st lens unit and a converging 2nd lens unit with the air separation therebetween being varied to effect zooming, or to the so-called two-component type, and many zoom objectives of this type have been proposed. The present applicant also has made proposals in Japanese Laid Open Patents Nos. Sho 53-132360 and 56-19022. In the former, the diverging front or 1st lens unit is made up of three lens members, and its power is somewhat strengthened to reduce the interval between the principal points of the front and rear lens units in the telephoto position to some extent. In this manner a valuable decrease in the diameter of the front lens members and the total length of the objective (the distance from the front vertex to the image plane) particularly in the wide angle position is achieved. In the latter, on the other hand, the front or 1st lens unit is imparted with linearly progressive axial movement when zooming, thus providing achievement of a valuable decrease in the total length of the objective in the telephoto position, although the power of the diverging front or 1st lens unit is weaker than that in the former zoom objective. To achieve a further advance in the compactness of the zoom lens system, in the former case, the power of the diverging front or 1st lens unit may be further strengthened with an advantage that the total length of the complete objective in the wide angle position can be further shortened. Conversely when in the telephoto position, however, the total length becomes progressively longer, finally exceeding that when in the wide angle position. In the latter case, on the other hand, it is advantageous at shortening the total length in the telephoto position. It is, however, in the wide angle position that the total length tends to increase and further that a large increase in the diameter of the front lens members will result. For a general solution of the problem of most effectively shortening the total length of the zoom objective of the two-component type, it is preferred to employ the rule of design that the total lengths in the wide angle and telephoto positions are equal to each other.